Fixture and method for attaching components

ABSTRACT

A fixture (10) for bonding multiple components together includes a bottom plate (20), a middle plate (22) and a top plate (24). The plates (20), (22) and (24) are aligned by dowels (26) and clamps (30). Bottom plate (20) has a rectangular pocket (32) for holding heat sink (14) and alignment pins (34) for locating plastic pin grid array (PPGA) package (12) over the heat sink (14). An annular projection (40) covered with a conformal pad (42) extends from bottom surface (44) of the middle plate (22). Dowel (50) extends through openings (46) and (36) in the top and middle plates (24) and (22) to apply pressure to chip (16). A first spring (56) is mounted on the dowel (50) and compressed between the top plate (24) and a snap ring (58) to provide pressure from the dowel (50) on the chip (16). A second, larger diameter spring (60) is compressed between the middle plate (22) and the top plate (24) to provide pressure from the middle plate (22) on the PPGA (12). Cartridge heaters (62) are embedded in the bottom plate (20) to provide heat for epoxy or solder bonding. A thermocouple (64) monitors temperature at the bottom plate (20).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a fixture and method forattaching multiple components together at one time. More particularly,it relates to such a fixture and method which simplifies and improvesthe assembly of electronic components. Most especially, the inventionrelates to such a fixture and method which reduces the amount of timethat parts of the electronic component assembly are exposed to hightemperatures needed for epoxy cure or solder reflow by combining severalassembly steps which would normally occur sequentially into one step.The invention further relates to a method which will allow two parts tobe bonded together at a low clamping pressure to generate thinnerjoints. This method is usable separately or in combination with thefixture and method for attaching many components together at one time.

It is often desirable to have very thin adhesive joints between twoparts. As an example, the joint between an integrated circuit chip and aheat sink offers the best heat transfer when the joint is very thin.

2. Description of the Prior Art

Electronic parts are assembled conventionally in the following manner: Apackage is first attached to a substrate, slug or heat sink surface,referred to hereafter for convenience as a heat sink. After that, anintegrated circuit chip is attached to the heat sink. Both operationsinvolve high temperatures and significant assembly time.

A similar approach is used to manufacture a multi chip unit (MCU) forthe VAX 9000 computer system. For the MCU, several chips are bonded ontoa common surface at one time. No other parts are bonded at this time,nor are more mating parts bonded at the same time.

When two flat surfaces are squeezed together with a flowing adhesivebetween them, the pressure that builds up in the adhesive is highest inthe center. Joints of very large area have very large pressure gradientsin them because the adhesive must flow a long distance when it issqueezed. Therefore, large area joints are usually thicker than smallarea joints. To make bonds thin, conventional techniques require highnormal pressures to squeeze the joint together.

Escape paths are used in the manufacture of wooden parts, where the gluethat forms a joint travels through a small channel. A wooden dowel thatis pressed into a hole filled with glue will often have grooves runningalong its axis to allow glue to escape.

Some epoxy films are made with a support matrix of fiberglass that, bythe nature of its shape, forms indentations, but not perforations.Manufacturers usually make an effort to reduce these indentations.

A need therefore exists for improvement in component attachment fixturesand methods, and in methods for producing thin adhesive joints.

SUMMARY OF THE INVENTION

A fixture for attaching multiple components together at one time inaccordance with this invention has a base member configured to hold thecomponents in position for assembly with an adhesive material thatbecomes fluid with application of heat between the components. A firstmeans applies pressure to urge a first pair of the components together.A second means applies pressure, independent of the pressure of thefirst means, to urge a second pair of the components together. A meansheats the components to a sufficient temperature to make the adhesivematerial fluid while pressure is applied from the first and second meansfor applying pressure.

A method for attaching multiple components together at one time inaccordance with this invention includes positioning the components forassembly with an adhesive material that becomes fluid with applicationof heat between the components. A first pressure is applied to urge afirst pair of the components together. A second pressure, independent ofthe first pressure, is applied to urge a second pair of the componentstogether. The components are heated to a sufficient temperature to makethe adhesive material fluid while the first and second pressures areapplied.

A structure for bonding a first component to a second component includesa layer of adhesive material which becomes fluid when heated positionedbetween the first component and the second component. Cavities arepositioned adjacent to the adhesive material to receive a portion of thefluid adhesive material.

A method for producing thin adhesive joints in accordance with thisinvention includes providing a layer of adhesive material which becomesfluid when heated between a first component and a second component.Cavities are positioned adjacent to the adhesive material to receive aportion of the fluid adhesive material. Pressure is applied between thefirst and second components. The layer of adhesive material is heateduntil it becomes fluid. The portion of the fluid adhesive material isallowed to flow into the regions.

The attainment of the foregoing and related advantages and features ofthe invention should be more readily apparent to those skilled in theart, after review of the following more detailed description of theinvention, taken together with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded side view of a fixture for attaching multiplecomponents together at one time in accordance with the invention.

FIG. 2 is a cross section view of the fixture in FIG. 1 in use.

FIG. 3 is a plan view of a portion of a second embodiment of a fixturein accordance with the invention.

FIG. 4 is a perspective view of a first assembly during practice of afirst embodiment of a process for forming thin joints in accordance withthe invention.

FIG. 5 is a perspective view similar to FIG. 4, but of a second assemblyduring practice of a second embodiment of a process for forming thinjoints in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, more particularly to FIGS. 1 and 2, thereis shown a fixture 10 for attaching multiple components together. Theelectronic components to be attached together with the fixture 10include, by way of example, a plastic pin grid array (PPGA) 12 to befastened to a metal heat sink 14, and an integrated circuit chip 16 tobe fastened to the heat sink 14. Adhesive layer 18, which is eithersolder or epoxy, is used for this purpose. While the same material isused for fastening both the PPGA 12 and the chip 16 to the heat sink 14,separate control over pressure applied to the PPGA 12 and the chip 16during the formation of their bonds to the heat sink 14 is requiredbecause of the different nature of the components and their area ofcontact with the heat sink 14.

The fixture 10 includes a bottom plate 20, a middle plate 22 and a topplate 24. The plates, 20, 22 and 24 are aligned by dowels 26. The dowels26 are fixedly attached to the bottom plate 20 and pass through dowelbushings 28 in the middle and top plates 22 and 24. Clamps 30 tightenedon the dowels 26 hold the top plate 24 in place. Bottom plate 20 has arectangular pocket 32 for holding the heat sink 14 and alignment pins 34for locating the PPGA 12 over the heat sink 14. The middle plate 22 hasa centrally disposed opening 36 over the pocket 32 on the bottom plate20, with a dowel bushing 38 in the opening 36. An annular projection 40covered with a conformal pad 42 extends from bottom surface 44 of themiddle plate 22. The top plate 24 also has a centrally disposed opening46 containing a dowel bushing 48 over the opening 36.

Dowel 50 extends through the openings 46 and 36 to apply pressure to thechip 16. End 52 of the dowel 50 has a conformal pad 54 for engaging thechip 16. A first spring 56 is mounted on the dowel 50 and compressedbetween the top plate 24 and a snap ring 58 to provide pressure from thedowel 50 on the chip 16. A second, larger diameter spring 60 iscompressed between the middle plate 22 and the top plate 24 to providepressure from the middle plate 22 on the PPGA 12.

Cartridge heaters 62 are embedded in the bottom plate 20 to provide heatfor the epoxy or solder bonding. A thermocouple 64 monitors temperatureat the bottom plate 20.

In use of the fixture 10, the epoxy or solder is applied to the surfaceof the heat sink 14. Various methods of application may be used, such assolder paste screening, solder preforms, solder dipping or coating,epoxy paste screening, paste dispensing and epoxy film preforms. Theheat sink 14 is placed in the locating pocket 32 of the bottom plate 20.The PPGA package 12 is placed over the heat sink 14, and is preciselylocated by the locating pins 34. The integrated circuit chip 16 isplaced on the heat sink 14 and is precisely located by features on thepackage 12 or on the fixture 10 (not shown). Other components, such ascapacitors, are placed in position.

Pressure is applied to the various components by the top plate 24 of thefixture 10 when/as it is slid down the dowels 26. The dowel 50, drivenby spring 56, applies pressure to the chip 16. The middle plate 22,driven by the spring 60, applies pressure to the PPGA package 12. Otherparts of the fixture 10 (not shown), each having their own springmechanism, may apply pressure to other components, such as capacitors.The pressure squeezes the parts into the soft epoxy or reflowing solder.Various spring types or snap ring locations can be used to adjust thepressures provided by the fixture 10 on different components beingassembled using the fixture.

The fixture 10 and parts are now placed in the desired environment forcuring. That environment may be atmospheric, nitrogen, vacuum, or thelike. With the fixture assembled, the cartridge heaters 62 heat theassembly up to a desired temperature. The thermocouple 64 is monitoredby a temperature controller (not shown), which supplies power to thecartridge heaters 62. After the required time at the requiredtemperature, the parts and fixture 10 are cooled down, and the assemblyis finished.

FIG. 3 shows a bottom plate 66 for a fixture similar to that of FIGS.1-2, but having an array 68 of pockets 70 for receiving heat sinks, sothat twelve of the packages as shown in FIG. 2 can be assembled at once.The middle plate and top plate for this fixture have correspondingarrays of the other elements of the fixture. Other than as shown anddescribed, the construction and operation of a fixture incorporating thebottom plate 66 is the same as in the FIGS. 1-2 embodiment.

FIG. 4 shows an integrated circuit chip 16, heat sink 78 and perforatedepoxy, thermoplastic sheet or solder preform 80, which is preferablyused with a fixture comparable to the fixture 10 to provide thinnerjoints in the assembly. The preform 80 is punched, drilled or cut toprovide perforations 82. The preform is then placed between the chip 16and the heat sink 78 in an evacuated chamber, so that air is not trappedin the voids defined by the perforations 82 and the flowing adhesive canmove into the void areas. As the parts are squeezed together in thefixture, the temperature of the assembly is increased to make theadhesive fluid. Since the pressure provided by the fixture causes theadhesive to flow, it flows in the path of least resistance, in thiscase, to the voids defined by the perforations 82. Releasing the vacuumprior to solidification will further collapse the voids. A pattern ofcircular perforations arranged at corners of equilateral triangles, withthe perforations being almost tangent to one another, representsproportionally the most area as perforations, up to about 22.5% of thesurface area of the preform.

FIG. 5 shows another embodiment of the method for producing thin joints,in which heat sink 90 has grooves 92 in its upper surface 94. A layer 96of an adhesive material, such as epoxy paste, thermoplastic preform orsolder is provided over the upper surface 94 of the heat sink 90. Whenintegrated circuit chip 14 is pressed down on the layer 96 and theassembly is heated, the pressure causes some of the resulting liquidadhesive to enter the grooves 92, thus producing a thinner adhesivelayer in the finished assembly. Instead of grooves, an array of wells inthe upper surface of the heat sink could be used. The pressure and heatcan be applied in an evacuated chamber as in the FIG. 4 embodiment toavoid trapped air in the grooves or wells. As in the FIG. 4 embodiment,a fixture as shown in FIGS. 1-3 is desirably used to hold the parts ofthe assembly, to apply the pressure, and to provide the heat.

By providing voids or spaces into which liquid adhesive will flow whenpressure is applied, the amount of pressure required to produce a jointof a given thickness is reduced. This is a substantial benefit fordelicate parts, such as used in electronic assemblies. Correspondingly,for a given amount of pressure, a thinner joint is produced. This isadvantageous in any application where improved heat transfer is requiredbetween two parts joined together with an adhesive.

The fixture and process of this invention allows assembly of such heatsensitive parts as electronic components with processes requiringelevated temperatures in a shorter time at high temperature. This is asignificant advantage, particularly when the elevated temperatureprocessing is carried out in the absence of air, specifically oxygen, asin a vacuum or nitrogen environment. This fixture and process saves thecost of ovens, reduces damage to the parts, such as intermetallic growthor material creep, and is faster than conventional processing.

It should be apparent to those skilled in the art that various changesin form and details of the invention as shown and described may be made.It is intended that such changes be included within the spirit and scopeof the claims appended hereto.

What is claimed is:
 1. A fixture for bonding multiple electronic packagecomponents of multiple types together at one time, which comprises abase member and a middle member configured to hold the multipleelectronic package components of multiple types in position forsimultaneous assembly with an adhesive material that becomes fluid withapplication of heat among the multiple electronic package components ofmultiple types, a top member positioned over said middle member, firstmeans including a first spring between said top member and a movableconnecting member passing through said middle member for applyingpressure to urge a first pair of the multiple electronic packagecomponents of multiple types together, second means including a secondspring between said middle member and said top member for applyingpressure, independent of the first means for applying pressure, to urgea second pair of the multiple electronic package components of multipletypes together, and means for simultaneously heating the multipleelectronic package components of multiple types to a sufficienttemperature to make the adhesive material fluid while pressure isapplied from said first and second means for applying pressure.
 2. Thefixture for bonding multiple electronic package components of multipletypes together at one time of claim 1 in combination with the multipleelectronic package components of the multiple types in which a componentof the first pair of the components is also a component of the secondpair of the components.
 3. The combination of claim 2 in which thecomponent of the first and second pair of components is a heat sink. 4.The fixture for bonding multiple electronic package components ofmultiple types together at one time of claim 1 in which said means forheating the components comprises at least one cartridge heater embeddedin said base member.
 5. The fixture for bonding multiple electronicpackage components of multiple types together at one time of claim 1additionally comprising means for monitoring temperature produced bysaid means for heating the components.
 6. The fixture for bondingmultiple electronic package components of multiple types together at onetime of claim 1 in combination with said first and second pairs of thecomponents, at least one layer of the adhesive material, and a pluralityof cavities positioned adjacent to the adhesive material to receive aportion of the fluid adhesive material.
 7. The combination of claim 6,with at least one of said first and second pairs of components havingthe adhesive material formed in a layer having perforations definingsaid plurality of cavities between the at least one of said first andsecond pairs of components.
 8. The combination of claim 6, with at leastone of said first and second pairs of components having a surface facingthe adhesive material having a plurality of indentations comprising saidplurality of cavities.
 9. The fixture for bonding multiple electronicpackage components of multiple types together at one time and componentcombination of claim 8 in which said plurality of indentations comprisegrooves.